Woo Audio WA7d Fireflies Headphone Amplifier and DAC Review John E. Johnson, Jr.August 10, 2015 Woo Audio, based in New York, builds a few power amplifiers, but lots of headphone amplifiers. In the current review, we take a look at the WA7d tubed headphone amplifier, with DAC, and solid state power supply. Headphone Amplifier Reviews